Active dendritic membrane properties of Xenopus larval spinal neurons analyzed with a whole cell soma voltage clamp

Voltage- and current-clamp measurements of inwardly directed currents were made from the somatic regions of Xenopus laevis spinal neurons. Current-vol tage (I-V) curves determined under voltage clamp, but not current clamp, we re able to indicate a negative slope conductance in neurons that showed str ong accommodating action potential responses to a constant current stimulat ion. Voltage-clamp I-V curves from repetitive firing neurons did not have a net negative slope conductance and had identical I-V plots under current c lamp. Frequency domain responses indicate negative slope conductances with different properties with or without tetrodotoxin, suggesting that both sod ium and calcium currents are present in these spinal neurons. The currents obtained from a voltage clamp of the somatic region were analyzed in terms of spatially controlled soma membrane currents and additional currents from dendritic potential responses. Linearized frequency domain analysis in com bination with both voltage- and current-cramp responses over a range of mem brane potentials was essential for an accurate determination of consistent neuronal model behavior. In essence, the data obtained at resting or hyperp olarized membrane potentials in the frequency domain were used to determine the electrotonic structure, while both the frequency and time domain data at depolarized potentials were required to characterize the voltage-depende nt channels. Finally, the dendritic and somatic membrane properties were us ed to reconstruct the action potential behavior and quantitatively predict the dependence of neuronal firing properties on electrotonic structure. The reconstructed action potentials reproduced the behavior of two broad distr ibutions of interneurons characterized by their degree of accommodation. Th ese studies suggest that in addition to the ionic conductances, electrotoni c structure is correlated with the action potential behavior of larval neur ons.